We are investigating the relationship between structure and function of hemeproteins. Myoglobin in the presence of hydrogen peroxide generates a ferryl species and a protein radical (Fe(IV)=O) and a protein radical. The radical was shown to reside on a His residue through site- specific mutagenesis. The Computer Graphics Laboratory is instrumental in these studies, allowing analysis of the three dimensional structure of myoglobin. In addition we have recently crystallized human heme oxygenase, the rate limitingenzyme in heme degradation. The data sets are close to being solved and on deposition of the coordinates the CGL will be instrumental in analyzing residues critical for binding and catalysis.